The present invention relates to apparatus and systems for gas-assist injection molding and more particularly to shut-off valves for gas-assist injection molding systems.
There are many processes and techniques today for gas-assisted injection molding. Gas-assisted injection molding processes have added flexibility to the design and manufacture of plastic parts with their ability to produce partially hollow, light-weight, rigid parts with minimal sink marks and less tendency to warp. These processes can reduce material requirements, as well as equipment costs and cycle time, and thus have advantages over conventional injection molding processes and techniques in many applications.
In general, gas-assisted injection molding systems utilize a fluid or gas, such as nitrogen, under pressure to expand the plastic material in the mold and conform it to the mold cavity details. The fluid or gas can be introduced into the mold in several way, such as through a bushing, nozzle, or machine nozzle into one or more cavities, or at one or more locations.
In typical operation, a “shot” or quantity of plastic material is injected into the mold cavity, then pressurized gas is introduced which causes the plastic to expand against the walls of the cavity. The gas pressure is maintained until the plastic material has cooled sufficiently to be self-supporting, whereupon the gas is vented, the mold is opened, and the plastic article is removed and/or ejected from the mold cavity.
With gas-assisted injection molding techniques, care must be taken to prevent the pressurized gas that is injected into the mold cavity from being directed into the bushing, runner system, or plastic injection machine.
Typically, a shut-off valve of some type is positioned in the machine nozzle or adjacent the machine nozzle in order to positively shut off the flow of plastic into the mold cavity and at the same time prevent back flow of plastic and/or gas into the plastic injection molding system. Known systems, however, typically require an external power source and a complicated power system, such as hydraulic, pneumatic, or electrical system with various electronic or mechanical controls. These systems are typically complex and expensive and require additional setup and programming time and expense.